Add to my last:
I setted "$grid['rowNum'] = 20;" but I see only 15 rows and the total is for 20 rows.
How I do to see 20 rows?

I don't fully understand your query.
However, you can set height to fixed pixels to get exact 20 row.

e.g.
$grid['height'] = '200';

When you set scroll = true, it will then remove the paging and load more records when you scroll to last record.
You must have more than 'rowNum' records to use scroll function.

In my database I have 21 rows.
Without the scroll active I see 20 rows in the first page of the grid and, obviously, 1 row in the 2nd page.
The sum that I put in the footer row shows ever the total of the page, not the general total.
With the scroll active however I can have, scrolling the grid until the end, the general total.
But, also if i put the rownum = 20, I have, in the first page, the sum of the first 20 rows but I see only the first 15 rows !!!
If I set height=460 (to see 20 rows) in the 2nd page I have the same height but only one row !!
Thanks
Massimo Gagliardi

Erratum to my previous.
If I set height = 460 (to see 20 lines), if I filter the result I will have the same height, but, for example, only one row !!

The height of the grid is not dynamic based on records.
You can set it once when configuring grid settings.